Thermoelectric effects in miniature permanent magnet probes used for velocity measurement in flowing sodium

Abstract

Velocities in liquid sodium at temperatures of 300°C and above have been measured using miniature magnet probes for two types of turbulent shear flow. Heated and unheated jets were studied using a high-sensitivity probe with an output of 48 μV·s/m. Buoyancy-influenced pipe flows were investigated with less sensitive, but more robust, probes generating about 15 μV·s/m. All velocity profiles measured with the probes were asymmetric, whether for heated flow or isothermal flow. Skew in the profiles was caused by the zero offsets of the probes changing with position, and an analysis of probe behavior suggests that the changes were thermoelectric effects due to inhomogeneities in the thermocouple materials forming the probe electrodes. Furthermore, the analysis shows that calibration offsets may be related to the apparent temperature difference across the probe indicated by the thermocouples when the probe is operating in isothermal conditions. The variation of zero offset with position seemed to be most severe when the probes were close to walls at the limits of their travel. The accurate determination of zero offsets for miniature magnet probes requires careful probe calibration in isothermal flow. Any small temperature difference across a probe during this process will produce errors in the signal. Similarly, temperature differences produced by turbulence during normal operation influence the results, particularly for probes of low sensitivity. This effect was seen during the present investigation in scatter in the velocity profiles for pipe flows, particularly at high test section powers and in downward flow. High heat fluxes produce correspondingly large temperature fluctuations; in downward flow, turbulence levels are increased by the buoyancy forces opposing the motion.